For successful use in thermoforming and molding applications polymer compositions must possess a good balance of a number of important properties. One important property requirement for such use is good Theological characteristics such as processability and/or high flowability. As part and mold designs become more intricate and detailed such as, for example, where embossed-type printing or patterns are desired on the face of finished parts, even better flow properties are required to completely fill the mold, avoid short-shots and thereby fabricate high quality parts. Good processability is required to prevent melt fracture and/or insure finished parts have smooth, defect-free surfaces. Furthermore, in contrast to the flow properties required for intricate mold design having relatively low flow length to wall thickness (L/T) ratio (e.g., about 100:1), for thinwall applications, which pertains to downgauged parts and goods having L/T ratios greater than 180:1, high flowability is particularly critical. Injection molded promotional cups provided as give-a-ways at fast-food restaurants is an common example of a thinwall application requiring exceptional polymer composition rheological characteristics well beyond those ordinarily required for conventional molding applications.
Good impact performance is another important polymer composition property required to insure successful use in thermoforming and molding applications. Thermoformed and molded goods intended, for example, for durable and storage use must have a level of impact and abuse resistance that insures long service life and repeated use. For thermoformed and molded containers used, for example, for refrigerated foodstuffs, good low temperature impact performance is also an important polymer composition requirement.
Good topload strength which pertains to high modulus, dimensional stability and compressive strength is still another important property requirement. Good topload strength permits thermoformed and molded goods to be conveniently stacked upon one another without having the goods deform or the stack itself become unstable. Good topload load strength also prevents containers from bulging or becoming unstable when filled with dense items or liquids such as, for example, a tall (e.g., 16-ounce) drinking cup filled to the brim with a beverage or an one-gallon tub filled with ice cream.
Good melt strength is another important property particular to thermoforming. As such, successful thinwall thermoforming requires polymer compositions that have good flowability and good melt strength.
In meeting the various property demands for successful thinwall thermoforming and molding, flow, melt strength, impact and topload properties must be carefully balanced. Structural polymer composition properties (such as, for example, density, branching and molecular weight) which directly affect one of the important properties can inversely and/or adversely affect one or more other properties. For example, it is well known that polyolefin compositions characterized as having lower densities have improved impact resistance relative to other polyolefin compositions having higher densities. Conversely, a polyolefin composition having a higher density invariably has a higher modulus relative to a lower density polyolefin composition. As such, good impact resistance and high modulus (good topload strength) are generally considered to be mutually exclusive polyolefin polymer properties. As another mutually exclusive relationship pertaining to impact resistance, it is also known that polyolefin compositions having higher molecular weights generally have improved impact and abuse properties, however such compositions also generally have inferior flow properties relative to polyolefin compositions having lower molecular weights.
High flowability and good melt strength is another relationship which is generally considered to be mutually exclusive. Whereas low molecular weights are generally required for high flowability, conversely, high molecular weights are generally required for good melt strength.
Although various polymer compositions are used in thinwall applications, a need presently exists for high flow or high melt strength compositions which exhibit substantially improved impact properties while retaining good topload strengths. In particular, there is a need for a polyolefin component polymer which at minor addition amounts effectively and substantially impact modifies high flow polyolefins also characterized as having good topload strength.